Species responses to fire, climate and human impact at tree line in the Alps as evidenced by palaeo‐environmental records and a dynamic simulation model

Summary 1. We use high-resolution records of macroscopic charcoal and plant remains from sediments of a lake in the Swiss Alps (GouilleRion, 2343 m a.s.l.) to reconstruct local fire variability and vegeta- tion dynamics over the last 12 000 years. 2. Species response to fire variability and to summer temperature was obtained by combining regression analyses between contiguous series of plant macrofossils, macroscopic charcoal and an available reconstruction of past summer temperature. 3. With a dynamic landscape vegetation model (LandClim), we simulated fire regimes using two levels of ignition frequency and moisture availability to disentangle the role of climate vs. humans on fire occurrence. The simulation results show that human disturbance was relevant in control- ling the fire variability and are in agreement with pollen evidence of human impact from previous studies from GouilleRion. 4. Our results show that fire is a natural disturbance agent in the tree line ecotone. Biomass avail- ability controlled the fire regime until increased land use and anthropogenic fire during the past 4000 years changed species composition and vegetation structure close to the tree line. 5. Important species at the tree line ecotone such as Pinus cembra greatly benefitted from periods with temperature above the modern mean July temperature, if anthropogenic fire disturbance was not too severe, such as during the Bronze Age (c. 4000 cal. yearsbp). 6. When mean July temperatures were lower than modern mean July values, Juniperus nana and Larix decidua were at an advantage over P. cembra. With increasing anthropogenic fire, open lands with J. nana replaced L. decidua and P. cembra forest stands. 7. Synthesis. Fire activity was low to moderate during the early and mid-Holocene. Intensified land use coupled with fire occurrence since the Bronze Age (c. 4000 cal. years bp) had a larger impact on species composition near the tree line than climate change. Although climate change will alter vege- tation composition, future dynamics of mountain forests will be co-determined by anthropogenic fire. For example, high fire variability may impede upslope establishment of forests in response to climatic warming as expected for this century, with serious implications for forest diversity.